Rowland



Jan. 24, 1956 L. ROWLAND ELECTRIC HEATING ELEMENTS 2 Sheets-Sheet 1Filed June 15, 1953 Fig. 1.

/NVENTOR ATTORNEYS Jan. 24, 1956 Filed June 15, 1953 L. ROWLAND ELECTRICHEATING ELEMENTS 2 Sheets-Sheet 2 E55. F1Jg5A. Fig.6. EgbA.

INVENTUR ATTORNEYJ United States Patent Ofiiice ELECTRIC HEATINGELEMENTS Leonard Rowland, Leicester,

bourn Aero Components England, a British company England, assignor toHo- Limited, Rochester, Kent,

This invention relates to electric heating elements, and has particularreference to heating elements of cord or ribbon form which are primarilyintended for mounting on refractory formers or equivalent supports.Elements of this kind are frequently employed in connection heaters anddrying machines and have hitherto consisted of strands of wire, usuallyof a nickel chrome alloy. This application is a continuation-in-part ofmy patent application Serial No. 247,961, filed September 24-, 1951, nowabandoned.

When nickel chrome Wires are employed to generate black heat it is foundthat due to repeated contraction and expansion the wires take on apermanent set with the result that they sag into mutual contact and thusshortcircuit the electrical heating circuit, thereby destroying theelement as a whole. In order to overcome this defect the ends of theWires are spring loaded, but this expedient is not entirely satisfactoryand moreover it adds greatly to the cost of the element.

The object of the present invention is to provide novel electricalresistance elements which retain substantially the forms in which theyare initially made and mounted, so that the possibility of sagging orother deformation likely to cause short-circuiting between adjacentwires or between different portions of the same wire is eliminated.

With the above object in view the invention provides a flexibleelectrical resistance heating element comprised of a number of strandsof heat-resisting electrically-insulating substantially inextensiblematerial, such as fibre glass, and at least one strand of wire, such asnickel chrome wire, said element being made by braiding together saidstrands of electrical resistance wire and substantially inextensibleinsulating material in such a manner that said strands of insulatingmaterial separate said wire from short-circuiting self contact and thatportions of said resistance wire, of which portions are exposed at orare disposed near the surface of the cord or ribbon element, is free toexpand and contract in relation to the substantially inextensiblematerial. If one or more wire strands are incorporated in the braidedelement, the wire or each wire is separated by the strands ofheat-resisting electrically insulating material from another part of thesame wire or from another or other wires in the same braided element,and where two or more elements are mounted on a common former or othersupport the wires in said plurality of adjacent elements are maintainedin substantially their initial positions of assembly.

For some applications, particularly where the element is encased so thatthere is no risk of contact therewith, it is suitable to use an elementin which portions of the resistance wire are exposed at intervals on thesurface of the element, so that the heat generated is quicklytransmitted to the surrounding air and at the same time heat-resistingelectrically-insulating material, such as fibre glass, is prevented fromreaching too high a temperature.

The heat-resisting electrically-insulating material may be composed ofone or a combination of materials and by reason of its inherentsubstantially inextensible nature 2,732,479 Patented Jan. 24, 1956 maybe mounted under tension, of adjacently disposed lengths of the flexibleheating element, whether of cord or ribbon form, allowing the exposedportions of resistance wire to make short-circuiting contact.

A further object of the present invention is to provide a flexibleheating element of the kind described which is so formed as effectivelyto protect the resistance wire, so that in the finished product the wireis not exposed. Consequently, even if lengths of the heating element docontact one another or a conductive member bridges spaced apart lengthsof the element an electric shock cannot be experienced.

With this further object in view the present invention provides also aflexible heating element as above defined, characterised in that acovering of flexible heat-resisting electrically-insulating material, toshield or envelop the resistance wire which would otherwise be exposedto view, is applied. This covering may be applied in the form of acovering all over the heating element having exposed portions ofresistance wire on its surface, or alternatively the wire may be coveredbefore its incorporation in the heating element, or, if desired andpracticable, both of the forms of covering may be used on the sameelement.

The covering material may be of the same kind as the insulating materialwhich may constitute the greater part of the heating cord or ribbon.That is, the cord or ribbon may be composed wholly of substantiallyinextensible material, such as fibre glass, or composed mainly ofstrands or threads, such as strands of spun asbestos, with a coreconstituted by a single thread or a group of threads of a material whichis substantially inextensible. The covering may be of the same bulk orbody material, that is, of asbestos strands, or it could be composed offibre glass, especially when this substantially inextensible material isused for a braided element.

In the accompanying drawings which illustrate a number of embodiments ofthe invention,

Figures 1 and 1A show a ribbon braid with two parallel electricalresistance wires,

Figures 2 and 2A showing another ribbon braid with a single electricalresistance wire in zig-zag form,

Figures 3 and 3A showing a cord braid incorporating a single electricalresistance wire,

Figure 4 shows the construction ment which is covered all over,

Figure 5 is an elevation of another braided cord element in whichprecovered resistance wire is used, Figure 5A being an enlarged view ofthis cord element, and

Figure 6 shows the same mode of construction, that is, the use ofprecovered wire, applied to a ribbon type of element, Figure 6A being anenlarged view of this ribbon element.

In the embodiment of the invention shown in Figures 1 and 1A the heatingelement consists of a flat braid composed of strands 1 of fibre glass,and of two longitudinal strands 2 of nickel chrome steel wire. A knownform of braiding machine is employed and in this particular embodimenteach of the two resistance wires is brought up through the centre of thesecond so-called horn gear from each edge, whilst all the other horngears feed a single strand of fibre glass. The longitudinal strands offibre glass keep the resistance wires apart, and so prevent shorting.The exposure of the wires at the surface, that is, on each face of thefiat braid, as previously referred to, is obtained by omitting a numberof bobbins at equally spaced positions in the braiding machine. Thisomission of bobbins also tends to make an open plait, so that the aircan pass through the braid during heating operations.

The larger the braiding machine, the wider the braid produced andaccordingly a larger number of longitudinal wires can then be employed.

so that there is no risk of a braided cord ele- An alternative method isto substitute a bobbin of nickel chrome wire for one of the fibre glass,in which case a single strand 1 of electrical resistance wire may bemade to appear in a zig-zag pattern on the braid, as shown in Figures 2and 2A, and this strand will be exposed on both surfaces of the braid.

If desired the braid may be made on a round or cord machine in which anumber of bobbins running in one direction form individual spirals onthe outside surface of the braid. By including alternate bobbins offibre glass, these spirals of wire will be insulated from each other,and so form a uniform round heating surface. By way of example, Figures3 and 3A show a cord braid in which a single strand of electricalresistance wire 1 is braided with a number of strands 2 of fibre glassso that the wire 1 is exposed intermittently along a spiral path. Ifdesired the exposed portions of wire 1 may be looped to stand outslightly from the surface of the cord.

The covered element illustrated in Figure 4 comprises a number ofuntwisted strands of fibre glass 1 encased in a braided layer of fibreglass strands 2 and a single resistance wire 3. This flexible .core isof the same form illustrated in Figures 3 and 3A. The all-over coveringin this case consists of a braided sleeve or sheathing 4 of fibre glassstrands. the resistance wire 3 near the sheathing 4 some of the strands2 of the core which normally would be used to make a uniform layer offibre glass are omitted and in the braiding operation this is providedfor by non-use of certain bobbin positions in the braiding machine asdescribed above. The sleeve or sheathing 4 is' applied to the core bybraiding fibre glass strands therearound, so that the sleeve orsheathing 4 closely envelops the core and cannot be easily strippedtherefrom. At the same time the sleeve or sheathing 4 is not so tightlyapplied as to interfere with free expansion and contraction of theresistance wire 3; that is, the sleeve or sheathing 4 is not madeinextensible in character.

The alternative mode of covering portions of resistance wire 3 whichwould nonnally be exposed at the surface of the element is byprecovering the wire 3 with a layer 3a of insulating material, such asfibre glass, as shown in the cord element illustrated in Figures 5 and5A and then incorporating this precovered wire in a braided cord elementas described with reference to Figures 3 and 3A.

This mode of covering the resistance wire is also readily applicable toa ribbon type of heating element as illustrated in Figures 6 and 6A.

Since the wire is braided into the glass fibre material, which ispractically inextensible, the contraction and ex;- pansion .ofthe wirehas little effect, so. that spring supports are unnecessary.

In order to dispose a sufficient amount of The length of braid requiredto generate any given temperature will depend, of course, on the voltageand resistance of the wire, and can be calculated according to Ohms law.

I claim:

1. A flexible braided heating element of cord form comprising a straightcore of heat-insulating material, a slightly extensible braidedsheathing of fibre glass, and an intermediate layer composed of strandsof fibre glass braided together with at least one electrical resistancewire in such a manner that the strands of fibre glass in saidintermediate layer separate the wire from self short-circuiting contactand that short portions of the wire are located in spiral formationalong the outside of said intermediate layer and thus disposed near thesurface of the cord element, being separated from said surface andconcealed solely by the thickness of the braided fibre glass sheathing,said sheathing permitting expansion of said wire when said wire becomesheated.

2. A braided heating element as claimed in claim 1, wherein the corecomprises a number of strands of fibre glass.

3. A flexible heating element comprising essentially a substantiallyinextensible flexible support at least one electrical resistance wirecarried by said support and a slightly extensible enclosing sleeve ofbraided heat-resisting electrically-insulating fibrous material, saidsupport being constituted by a core of straight strands ofheat-resisting electrically-insulating fibrous material and upon saidcore a body of braided strands of heat-resisting electrically-insulatingfibrous material, and said at least one electrical resistance wire beingbraided into the outer layer of said body strands to guide it in asinuous path to separate successive portions from self short-circuitingcontact and to expose short portions of said resistance wire along thesurface of said outer layer of body strands, so that said portions ofwire are concealed solely by the slightly extensible enclosing braidedsleeve.

4. A flexible heating element as claimed in claim 3., wherein the coreand body of the support and the enclosing sleeve are all composed ofstrands of fibre glass.

References Cited in the file of this patent UNITED STATES PATENTS2,345,300 Simpson et al Mar. 28, 1944 2,385,577 Jacob V Sept. 25,19452,396,099 Hartwell Mar. 5, 1,946 2,451,839 Lemon Oct. 19, 1,9482,496,279 Ely et al Feb. 7, 1.950 2,610,286 Cox Sept. 9, 1952

